The invention relates to a method and an apparatus for the production of thermoplastic caprolactam, i.e., nylon-6, containing molding compositions by hydrolytic polymerization of caprolactam in a plurality of autoclaves at pressures of up to 20 bar and temperatures up to at least 240.degree. C. in the presence of water and a weak acid, whereafter the molten mass is transferred out of the autoclaves into an intermediate vessel to which vacuum is applied for continued polymerization and for the removal of volatile components such as water, monomeric caprolactam and oligomers.
The above-mentioned method step with the use of a single autoclave are known from DD-PS 4,735, which relates to a method for the production of linear polyamides in two discontinuous steps. An extrusion or a processing into molding compositions is not provided.
Polyamide molding compositions are normally produced in the following way. In the first step, the caprolactam is melted down and either polymerized entirely continuously in a VK-tube or discontinuously in an autoclave, whereat other polyamideformers, for instance lauryllactam, or adipic acid or sebacic acid and the equivalent quantity of hexamethylene diamine are added. The caprolactam containing monomer mixture is thereby under nitrogen atmosphere heated to 220.degree. to 280.degree. C., whereby the pressure can increase to 20 bar. After 6 to 8 hours, the polyamide molten mass at 220.degree. to 250.degree. C. is extruded into strands, quenched and granulated. The polycaprolactam containing raw polymer obtained in this way contains 10 to 11% by weight monomeric caprolactam in relation to the total quantity of polymeric caprolactam. By means of washing the polymer with a 3 to 10-fold quantity of hot water in relation to the polyamide, the monomeric and low molecular weight and oligomeric portions are extracted. The raw polymer saturated with moisture is subsequently dried.
The energy requirement of the above-mentioned multiple-step method, among others, is very unfavorable, as is shown by the following. In order to melt down and to heat 1 kg caprolactam to the aforementioned temperatures, there are required approximately 300 kcal. During the polymerization, a reaction heat of approximately 100 kcal/kg is released, so that for the polymerization, there are required approximately 200 kcal/kg caprolactam. For the extraction of 1000 kg polycaprolactam, there are required 2800 l boiling-hot water in an energy-conserving countercurrent process. The polycaprolactam to be extracted is thereby at first treated with caprolactam containing washing liquor and finally with pure water. For heating this quantity of water from 12.degree. to 100.degree. C., there are required 88 kcal/l, that is, for the 2800 l, 0.10234 kWh.times.2800 =286.5 kWh. For recovering the caprolactam, the aqueous solution has to be distilled. This requires 2000 kWh, which is calculated as follows: the heat of evaporation of 1 kg water is 539 kcal, which is equal to 0.6268 kWh, that is, for the 2800 l, 0.6268 kWh.times.2800=1755 kWh. For cooling down again the distilled water from 100.degree. C. to 25.degree. C., there is required either costly fresh water or an additional 75 kcal.times.2800 l=244 kWh and for the distillation of 100 kg caprolactam at 12 to 15 hPa in a thin-layer evaporator another 35 kWh, so that for the washing and extracting of 1000 kg polycaprolactam containing raw polymer, for the distillation of the lactam containing water and the caprolactam, there are required a total of 2500 to 2800 kWh, depending on the magnitude of the heat losses by radiation. Without the use of the countercurrent principle in the extraction, the energy consumption is two to three times higher (see Kunststoffhandbuch, Munich, 1966, volume VI, p. 190 to 194; DE PS 25 03 308).
After the extraction, further energy is required for drying the granules. The most economical way for doing this is in a vacuum dryer within 36 hours at 70.degree. to 90.degree. C. For drying 1000 kg polyamide, there are required at least 270 kWh, because for the evaporation of the water, there are required 539 kcal/kg=0.63 kWh/kg. The washed and pre-dried polyamide contains approximately 14% water; 1000 kg thus contain 140 kg water, so that for drying to a water content below 0.1%, there are required 140.times.0.63=88.2 kWh. In addition, there is also required driving energy for the vacuum tumble-dryer, which requires at least 3 kWh/1000 kg polyamide.times.36 hours=108 kWh
For the vacuum pump, there are required another 36.times.2 kWh =72 kWh/1000 kg. This gives a total of 268 kWh, effectively 280 to 300 kWh due to heat losses and radiation. In order to obtain a marketable caprolactam containing thermoplastic molding material, the extracted and dried polyamide has to be melted down in an extruder, so that pigments, additives and, when required, fillers can be worked in. For this process, the empirically determined energy requirement is approximately 300 to 380 kWh/1000 kg nylon-6 molding material This value can be calculated from the heat of fusion for nylon-6=0.019445 kWh/kg and the heating and driving energy requirement of the extruder of 280 to 300 kWh/ton polyamide. Therefore, according to the conventional method, per ton of end product, there have to be expended a total of approximately 3050 to 3460 kWh/1000 kg nylon-6.
In individual cases, it has already been attempted to shorten this multiple-step process to effect energy savings (see EP - PS 0005 466). It has already been proposed to arrange behind the VK-tube a twin-shaft extruder and to make the method continuous. However, the extruder is proposed only as a worm evaporator as an alternative to the conventional thin-layer evaporators for the degasification. No other possibilities are provided. In addition, the therein intended use of a VK-tube does not permit any production of variants, but limits it to a pure polycaprolactam of relatively low melt viscosity. These materials are suitable for the production of monofilms, but not for the production of highly impact resistant or reinforced injection molded molding materials.